1. Field of the Invention
The present invention relates to an image building apparatus of the type in which both a sheet of printing medium and a strip of ink donor medium are displaced in the superimposed state in the area as defined between a printing head and a platen and thermal transferring is effected by a transferring coloring agent on the ink donor medium to the printing medium with the aid of the printing head and more particularly to an improvement of an image building apparatus of the above-mentioned type which is intended to design and construct the apparatus in smaller dimensions.
2. Description of the Prior Art
A heat sensitive ink transferring type recording apparatus with a thermal printing head serving as a printing head employed therefor is herein noted as a typical apparatus of the early mentioned type. This type of apparatus is generally constructed such that a sheet of printing medium (paper) is partially wound about the platen and an image is built on the printing medium by transferring coloring agents on a strip of ink donor medium (thermal transfer ribbon) to the printing medium with the aid of the thermal printing head while the ink donor medium is brought in pressure contact with the printing medium in the superimposed state. When multicolor transference is to be carried out, the printing medium is required to reciprocably move for each of plural coloring agents so as to effect transferring for all coloring agents.
However, it has been found that the conventional apparatus has a drawback that incorrect displacement of the printing medium away from the predetermined position takes place due to an occurrence of slippage of the printing medium along the outer surface of the platen at every time when transference is achieved for each of coloring agents while the printing medium is reciprocably displaced. To obviate the above-mentioned drawback, there is already made such a proposal that a pair of thrusting roller mechanisms are disposed at the positions located downstream and upstream of the thermal printing head as seen in the direction of transportation of the printing medium in order to assure that the printing medium is accurately displaced while it is firmly clamped between the thrusting roller mechanisms and the platen. When the pair of thrusting roller mechanisms are so constructed that they are always brought in pressure contact with the platen, it results that the printing medium is always caused to reciprocably move while it is clamped between the thrusting rollers and the platen. This leads to a problem of increasing a non-image building area on the printing medium where no image can be built (located at both the ends of the printing medium as seen in the transverse direction).
In view of the above-mentioned problem there has been lately made another proposal that a pair of thrusting rollers are so arranged that they are displaced toward and away from the platen in accordance with the reciprocable movement of the printing medium, thereby reducing the non-image building area on the printing medium. However, the apparatus according to this proposal has a problem that levers, shafts, resilient members or the like components are required and thereby the apparatus becomes has larger dimensions.